1. Field of the Invention
The present invention relates to a presensitized plate for lithographic printing and to a lithographic printing method which uses the presensitized plate. More specifically, the present invention relates to a presensitized plate which, by being exposed to an infrared laser scanned over it based on digital signals from a computer or the like, can be made directly into a lithographic printing plate, and relates also to a lithographic printing method in which the foregoing presensitized plate is directly developed and printed on a printing press without passing through a development step.
2. Description of the Related Art
Lithographic printing plates are generally composed of oleophilic image areas which are receptive to ink during the printing operation and hydrophilic non-image areas which are receptive to dampening water. Lithographic printing is a process that utilizes the mutual repellence between water and oil-based inks by having the oleophilic image areas of the plate serve as ink-receptive areas and having the hydrophilic non-image areas serve as water-receptive areas (non-ink receptive areas), thus creating differences in the ability of ink to adhere to the surface of the plate and allowing the ink to be deposited only in the oleophilic image areas. The ink that has been selectively deposited on the plate is then transferred to a printing substrate such as paper.
Presensitized plates composed of an oleophilic photosensitive resin layer (image recording layer) on a hydrophilic support are widely used to make such lithographic printing plates. Generally, the lithographic printing plate is obtained by a platemaking process in which the presensitized plate is exposed to light through an original on lith film or the like. Next, the image recording layer is left intact in image areas but is dissolved and removed with an alkaline developer or an organic solvent in non-image areas, thereby revealing the surface of the hydrophilic support.
Platemaking operations with prior-art presensitized plates have required, following light exposure, a step in which the non-image areas are dissolved and removed, typically with a developer suitable for the image recording layer. One challenge has been how to simplify or eliminate altogether such wet development carried out as an ancillary operation. In particular, the treatment of wastewater discharged in connection with wet treatment has become a major issue throughout the industry owing to concerns over the global environment, making the need for a solution to the above problem increasingly acute.
One simple platemaking process that has been devised in response to the above need is referred to as “on-machine development.” This involves the use of an image recording layer which allows non-image areas of the presensitized plate to be removed in an ordinary printing operation. Following exposure of the presensitized plate to light, the non-image areas are removed on the printing press, yielding a lithographic printing plate.
Exemplary on-machine development methods include techniques that use a presensitized plate having an image recording layer which can be dissolved or dispersed in dampening water, ink solvent or an emulsion of dampening water and ink; techniques that mechanically remove the image recording layer by bringing it into contact with the impression cylinder or blanket cylinder on the printing press; and techniques in which cohesive forces within the image recording layer or adhesive forces between the image recording layer and the support are weakened by the penetration of, for example, dampening water or ink solvent, following which the image recording layer is mechanically removed by contact with the impression cylinder or blanket cylinder.
In this specification, unless noted otherwise, “development step” refers to an operation in which, using an apparatus other than a printing press (typically an automated processor), the areas of the presensitized plate which have not been exposed with an infrared laser are brought into contact with a liquid (typically an alkaline developer) and removed, thereby revealing the surface of the hydrophilic support. Likewise, “on-machine development” refers herein to a process and operation in which, using a printing press, areas of the presensitized plate which have not been exposed with an infrared laser are brought into contact with a liquid (typically dampening water for printing) and removed, thus revealing the surface of the hydrophilic support.
However, when use is made of a prior-art image recording layer that utilizes ultraviolet light or visible light to record an image, because the image recording layer is not fixed even after exposure to light, it has been necessary to resort to a cumbersome process such as storing the exposed presensitized plate in a completely light-shielded state or under constant-temperature conditions before it is mounted on the printing press.
In recent years, the use of digitizing technology to electronically process, store and output image information using computers has become very widespread, and various new image output systems adapted to such digitizing technology have come into use. Most notably, these trends have given rise to computer-to-plate technology, in which digitized image data is carried on a highly convergent beam of radiation such as laser light which is scanned over a presensitized plate to expose it, thus enabling the direct production of a lithographic printing plate without relying on the use of lith film. One major technical challenge has been the development of presensitized plates suitable for computer-to-plate technology.
As already noted, the desire today for simpler platemaking operations which either involve dry development or are development-free has grown increasingly acute, both on account of concerns over the global environment and for compatibility with digitization.
Given the availability today of inexpensive high-output lasers such as semiconductor lasers and YAG lasers, methods which employ these high-output lasers as the image recording means show much promise because they involve the production of lithographic printing plates by scanning-type exposure which can readily be integrated with digitizing technology.
In prior-art platemaking processes, the imagewise exposure of a photosensitive presensitized plate is carried out at a low to moderate illuminance, and the image is recorded by imagewise changes in physical properties brought about by photochemical reactions within the image recording layer. By contrast, in methods that use the high-output lasers mentioned above, the region to be exposed is irradiated with a large amount of light energy for a very short period of time, the light energy is efficiently converted into thermal energy, and the heat triggers chemical changes, phase changes and changes in form or structure within the image recording layer. Such changes are used to record the image. Thus, the image information is input by light energy such as laser light, but the image is recorded using both light energy and reactions triggered by thermal energy. Recording techniques which make use of heat generated by such high power density exposure are generally referred to as “heat mode recording,” and the conversion of light energy to heat energy is generally called “photothermal conversion.”
The major advantages of platemaking methods that use heat mode recording are that the image recording layer is not sensitive to light at ordinary levels of illuminance such as indoor lighting, and that the image recorded with high-illuminance exposure does not need to be fixed. That is, the presensitized plates used in heat mode recording are not sensitive to indoor light prior to exposure and do not require the image to be fixed following exposure. It is therefore possible to have a printing system in which, for example, an image recording layer that can be rendered insoluble or soluble by exposure to light from a high-powered laser-is imagewise exposed, and the exposed layer is subsequently rendered into a lithographic printing plate in a platemaking operation carried out by on-machine development. In such a system, following exposure, the image incurs no effects even when exposed to ambient indoor light. Accordingly, there is some reason to believe that by using heat mode recording, presensitized plates suitable for use in on-machine development may be obtained.
Remarkable advances have been made recently in laser technology. In particular, it has become possible to easily acquire small, high-output semiconductor lasers and solid lasers which emit infrared light at wavelengths of 760 to 1200 nm. Such infrared lasers are extremely useful as recording light sources when making printing plates directly from digital data on a computer or the like.
However, in most photosensitive recording materials of practical use as image recording layers, the photosensitive wavelength is in the visible light range at wavelengths of 760 nm or less. Infrared lasers cannot be used for recording images on such materials. Accordingly, there exists a need for materials on which images can be recorded using infrared lasers.
In response to this need, JP 2938397 B (the term “JP XXXXXXX B” as used herein means a “Japanese patent”) describes a presensitized plate in which a hydrophilic support has provided thereon an image-forming layer composed of hydrophilic binders with hydrophobic thermoplastic polymer particles dispersed therein. JP 2938397 B describes the exposure of a presensitized plate using an infrared laser and the ensuing coalescence of hydrophobic thermoplastic polymer particles under the effect of heat to form an image, and how the plate can then be mounted onto the cylinder of a printing press and on-machine development carried out with dampening water and/or ink.
Although methods in which an image is formed in this way by the coalescence of small particles using thermal fusion alone exhibit good on-machine development properties, the exposed plate has a very low image strength (adhesion to the support) and thus a short press life.
JP 2002-287334 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”) describes a presensitized plate composed of a support on which has been provided a photosensitive layer containing an infrared absorber, a radical polymerization initiator and a polymerizable compound.
Methods that use polymerization/crosslinking reactions in this way have a relatively good image strength because of the high chemical bond density in the image areas. However, because both the press life and the polymerization efficiency (sensitivity) fall short of what is needed for practical purposes, such plates have yet to see commercial use.